This application claims the priority of Japanese Patent Application No. 5-212323 filed on Aug. 3, 1993, which is incorporated herein by reference.
1. Field of the invention
The present invention relates to a nonaqueous electrolyte battery and, more specifically, to improvement of positive electrode active material to improve the storage capability.
2. Description of the prior art
Lithium batteries (primary battery) have the advantages of 1 higher weight energy density compared to conventional alkali batteries, 2 achievement of high operating voltages of 3 to 4 V by selecting an appropriate positive electrode active material, and 3 good storage capability thanks to formation of a thin protective film having ionic conductivity on the surface of lithium negative electrode as caused by reaction with the electrolyte used (aprotic organic solution). Lithium batteries have, therefore, been widely used as power source for watches, electronic calculators, cameras, instruments and the like.
Among lithium batteries, manganese dioxide-lithium batteries, having manganese dioxide as a positive electrode active material, are currently produced in the largest volume, since they have better battery characteristics compared to other lithium batteries and are advantageous in raw material cost due to the low price of manganese oxide.
However, with conventional manganese dioxide-lithium batteries, the manganese oxide reacts with the electrolyte used during storage and the resulting reaction products further react with the lithium on the negative electrode side so that the batteries undergo self discharge (self discharge rate: about 1%/year). These batteries are therefore not provided with satisfactory storage capability.
As a result of an intensive study, the present inventors have found that the storage capability markedly improves when a fluorinate graphite having a specific fluorine content is added to manganese dioxide in a specific amount.
Japanese Patent Application Laid-open No. 88671/1982 proposes a nonaqueous electrolyte battery utilizing a mixture of manganese dioxide and a fluorinated graphite as a positive electrode active material. However, the fluorinated graphite used in the battery proposed by this application has a relatively low fluorine content. Thus, the compound is represented by formula (CyFx)n wherein y is 1 or 2 and x is not more than about 1.1. With respect to storage capability which the present invention intends to improve, this type battery is of the same level as the above mentioned conventional manganese dioxide-lithium batteries utilizing manganese-dioxide alone.